Techniques have been developed to improve thermal efficiency of thermal power generation as a measure for saving energy. To improve thermal efficiency of a gas turbine for power generation, it is effective to increase an inlet temperature of gas.
To increase an inlet temperature of gas, it is necessary to form stationary vanes, rotor blades, wall parts of a combustor, and the like of a gas turbine from heat-resistant members. Stationary vanes and rotor blades are made from heat-resistant metal but cannot bear such an elevated temperature, and thus thermal barrier coating (TBC) is formed on a substrate made from heat-resistant metal.
Thermal barrier coating includes a metallic bond layer and a ceramic layer applied in series on a substrate. A ceramic layer is formed on a metallic bond layer by thermal spraying, and is often made from a ZrO2-based material, yttria-stabilized zirconia (YSZ) in particular, which is a ZrO2 partially or fully stabilized by addition of Y2O3, for YSZ has a relatively low thermal conductivity and a relatively high thermal expansion rate among ceramic materials.
In recently developed gas turbines, an inlet temperature of gas gets higher than 1600° C. If rotor blades and stationary vanes are coated with thermal barrier coating including a ceramic layer formed from YSZ, there is a risk of separation of a part of the ceramic layer during operation of a gas turbine under a harsh operational condition of a temperature exceeding 1600° C., which may deteriorate thermal resistant property.
Furthermore, gas turbines having an even higher thermal efficiency have been developed in recent years, and in some of them an inlet temperature of gas of gets as high as 1700° C. In this case, the surface temperature of stationary vanes and rotor blades is assumed to increase to 1300° C. Thus, thermal barrier coating is required to have a thermal resistance and a barrier property improved even further.
Separation of a ceramic layer made from YSZ is due to insufficient crystallization stability of YSZ under a high-temperature environment, as well as insufficient durability of YSZ against a great thermal stress. In view of this, ceramic layers having an excellent crystallization stability under a high-temperature environment and a high thermal durability have been developed, including Yb2O3+ZrO2 (Patent Document 1), Dy2O3+ZrO2 (Patent Document 2), Er2O3+ZrO2 (Patent Document 3), and SmYbZr2O7 (Patent Document 4).
On the other hand, ceramic powders used as a material of ceramic layers are expensive, and it is desirable to reduce costs for producing ceramic layers.
In view of this, Patent Document 5 discloses a method of reusing particles of a ceramic spray powder that remain unattached to an object after being sprayed to the object. The reusing method includes a non-adhering powder recovery step of recovering particles including a ceramic spraying powder that remain unattached to an object as a recovered powder, a separating step of separating the recovered powder into a metal recovered powder including a metal spray powder and a ceramic recovered powder including a ceramic spray powder by classification, for instance, and a spray-powder reusing step of spraying the ceramic recovered powder obtained in the separating step to another object as a thermal spray powder.
Typically, a ceramic spray powder that remains unattached to an object has been discarded, but according to the reusing method disclosed in Patent Document 5, a ceramic spray powder that remains unattached to an object is reused, and thereby the amount of ceramic spray powder to be discarded decreases. As a result, it is possible to reduce the amount of a fresh ceramic spray powder to be purchased additionally, which reduces the total production cost for forming ceramic layers, in spite of additional costs required to reuse the ceramic spraying powder.